Delivery system for implantable medical device

ABSTRACT

A delivery device for implanting a medical device that includes an expandable fixation member adapted to fix the position of the medical device within a lumen of a human body. The delivery device has an inner shaft rotatably disposed in a tubular outer shaft. A retention member is secured to and rotatable with the inner shaft and has a free end and a retainer portion adapted to protrude outwardly through an exit aperture in the outer shaft to extend circumferentially about the exterior of the outer shaft. The fixation member of the medical device may be retained on the tubular shaft in a low profile configuration by the outwardly protruding retainer portion and may be released to expand upon retraction of the retainer portion in response to rotation of the inner shaft.

FIELD OF THE INVENTION

The invention relates to implantable medical sensors and fixation ofsuch sensors in body lumens.

BACKGROUND

Various implantable medical devices have been clinically implanted orproposed for therapeutically treating or monitoring one or morephysiological conditions of a patient. Such devices may be adapted tomonitor or treat conditions or functions relating to heart, muscle,nerve, brain, stomach, endocrine organs or other organs and theirrelated functions. Advances in design and manufacture of miniaturizedelectronic and sensing devices have enabled development of implantabledevices capable of therapeutic as well as diagnostic functions such aspacemakers, cardioverters, defibrillators, biochemical sensors, andpressure sensors, among others. Such devices may be associated withleads for electrical functions or may be wireless, with the ability totransmit data electronically either to another device implanted in thepatient or to another device located externally of the patient, or both.

Although implantation of some devices requires a surgical procedure(e.g., pacemakers, defibrillators, etc.) other devices may be smallenough to be delivered and placed at an intended deployment site in arelatively noninvasive manner, such as by a percutaneous deliverycatheter. Depending on the nature, function and intended deployment siteof the device, the manner in which the device is fixed in place andoriented in the body may affect the operation and accuracy of thedevice. Consequently, the means by which the device is fixed in place inthe body can be a significant factor in its performance and utility.

By way of illustrative example, implantable miniature sensors have beenproposed and used in blood vessels to measure directly the diastolic,systolic and mean blood pressures, as well as body temperature andcardiac output. Such direct in vivo measurement of hemodynamicparameters may provide significant information to clinicians tofacilitate diagnostic and therapeutic decisions. If linkedelectronically to another implanted therapeutic device (e.g., apacemaker), the data can be used to facilitate control of that device.Such sensors also, or alternatively, may be wirelessly linked to anexternal receiver. As one example, patients with chronic cardiovascularconditions, particularly patients suffering from chronic heart failure,may benefit from the use of implantable sensors adapted to monitor bloodpressures. Promising indications have been reported for using suchimplantable sensors. Accurate knowledge of a patient's hemodynamicparameters can inform the decision whether to admit the patient to thehospital or whether the patient's condition can be managed with othertherapies not requiring hospital admission. This is particularly so inconnection with measurements of the blood pressure in the pulmonaryartery that cannot be measured readily from an external location.Assessing a patient's pulmonary artery blood pressure is a criticalfactor in diagnosing the heart failure patient and determining how bestto manage the patient. Typically, blood pressure in the pulmonary arteryhas been determined by using a balloon-tipped pulmonary artery catheterhaving a pressure measurement function and sold under the trademarkSWAN-GANZ, which is inserted and navigated through the right side of thepatient's heart and the pulmonary valve into the pulmonary artery, aprocedure that requires hospitalization. It has been estimated thatthere are about five million patients in the United States who sufferfrom heart failure with approximately one million hospital admissionsper year to assess and treat the condition. It would be desirable toprovide a means by which such data could be obtained before admittingthe patient to the hospital as the patient may experience an improvedquality of life and it might avoid the necessity for and cost ofhospitalization.

It is among the general objects of the invention to provide a minimallyinvasive, improved means for controllably placing and supporting animplantable sensor within a body lumen in a position, location andsensor element orientation that facilitates the operation of the device,in which the means includes a fixation member to which the sensor ismounted to achieve these objects.

SUMMARY OF THE INVENTION

In accordance with the invention, an implantable sensor is attached to afixation member of wire-like construction that is expandable from a lowprofile configuration, in which a catheter can deliver it to thedeployment site in the vessel, to an expanded configuration in which itis deployed in the vessel in engagement with the vessel wall. Thefixation member may be formed from a highly resilient material,preferably one having superelastic properties and includes at least onelinear attachment strut and at least one self-expandable portion. Thesensor includes a housing with attachment elements adapted to receivethe attachment strut in a manner that fixes the position of the sensorrelative to the axis of the attachment strut and prevents the sensorhousing from rotating about the strut. The housing of the sensorincludes an elongate channel adapted to receive the attachment struttransversely. The channel may be defined in part by bendable tabs thatare plastically deformed over the inserted attachment strut to securethe sensor housing and strut together. In another aspect of theinvention the wire-like fixation member, including the expandableportion, is formed to lie in a single plane when expanded. The fixationmember is dimensioned with respect to the intended deployment site sothat when expanded it will engage the wall of the vessel atdiametrically opposed locations in the vessel sufficiently to secure thefixation member and sensor housing to which it is attached, in place. Inone embodiment of the invention the sensor housing may contain pressuresensing components including an externally exposed sensing element andis mounted to the fixation member such that, when the fixation member isdeployed, the sensing element of the sensor will face along a directiongenerally perpendicular to the plane of the fixation member, to faceinwardly toward the center of the vessel lumen and be exposed fully tothe pressure within the vessel. In another embodiment the fixationmember and sensor are arranged so that the sensing element facesgenerally parallel to the plane of the fixation member. The fixationmember also may be configured to position the sensor housing and,particularly, the sensing element, away from the vessel wall to lessenthe risk of turbulent flow through the vessel.

In a further aspect of the invention the fixation member includes atleast one continuous loop integral with the attachment strut with theloop being non-circular, preferably somewhat teardrop-shaped, havingnarrow and broad ends. The narrow end is toward the middle of thefixation member with the broader portion located at the end of thefixation member where it can be engaged by a delivery device. Thefixation member is formed from a superelastic material and the loop iscompressible to a low delivery profile shape of a pair of approximatelyparallel wires. The narrow end of the teardrop loop is adapted to avoidexcessive strain on that portion of the loop when the loop is compressedto its low profile, delivery shape.

In a further aspect of the invention a delivery device for the sensorassembly may include a catheter on which the sensor assembly is mountedin its low profile configuration. The assembly is retained on thecatheter shaft by a pair of longitudinally spaced, helical retentionelements, secured to a rotatable inner shaft contained in the cathetershaft. The sensor assembly is loaded so that each helical retentionelement has a free end that protrudes out of an exit aperture in thecatheter shaft. The protruding end wraps about one of the compactedloops of the sensor assembly and reenters the shaft through a secondaperture circumferentially spaced from the first. When the deliverydevice has been navigated to the intended deployment site the innershaft is rotated in a direction to withdraw the free ends into thecatheter shaft thus releasing the fixation member and enabling it toself-expand. The loops of the sensor assembly may be releasedsimultaneously or may be released sequentially to enable the clinicianto confirm proper placement of one of the loops before releasing theother. The arrangement enables the sensor assembly to be recaptured andrepositioned should that be indicated.

In another aspect of the invention a sensor housing may be secured tofixation members that have a tubular shape by forming the fixationmember to include at least one linear attachment strut that can bereceived transversely in the channel of the housing and by thenmechanically securing the strut in the channel.

It should be understood that although the invention is describedprincipally in the context of fixing a sensor in the pulmonary arterytree to measure blood pressure, the invention is not limited to use inthat context. The principles of the invention may be used to makeimplantable sensors assemblies adapted to measure and monitor any of avariety of physiological parameters.

DESCRIPTION OF THE DRAWINGS

The advantages, features and objects of the invention will beappreciated more fully from the following description and accompanyingdrawings in which:

FIG. 1 is a diagrammatic illustration of a human patient depicting thelocations of implantable medical devices including, for example, apacemaker or defibrillator and a wireless sensor assembly placed in thepulmonary artery of the patient;

FIG. 2 is an isometric illustration of one embodiment of a sensorassembly embodying the invention in an expanded configuration;

FIG. 3 is a plan view of the sensor assembly shown in FIG. 2;

FIG. 4 is an end view of the sensor assembly shown in FIG. 2 as seenalong the line 4-4 of FIG. 3;

FIG. 5 is a bottom view of the sensor assembly shown in FIG. 2 as seenalong the line 5-5 of FIG. 4;

FIG. 6 is an enlarged isometric illustration of the connection betweenthe attachment strut of the fixation member and the sensor housing;

FIG. 7 is an illustration of the battery portion of the housing showingthe arrangement of channel-defining tabs;

FIG. 8 is a diagrammatic illustration of one embodiment of a deliverydevice that may be used to deliver and deploy a sensor assembly having asingle plane fixation member;

FIG. 9A is an illustration of a portion of the delivery catheter of FIG.8, partly broken away and with the sensor assembly removed for clarity;

FIG. 9B is a diagrammatic illustration of the rotatable shaft of thedelivery device by which the sensor assembly is retained and released;

FIG. 9C is an enlarged isometric illustration of a portion A of thedelivery device of FIG. 9A, partly in section, showing a loop of thefixation member of the sensor assembly in its low profile configurationin readiness to be attached to the delivery device;

FIG. 9D is an illustration similar to FIG. 9C showing the deliverydevice with the shaft having been rotated to cause the helical coil tosecurely engage the fixation member of the sensor assembly to thedelivery shaft;

FIG. 9E is a sectional illustration as seen along the line 9E-9E of FIG.9D;

FIG. 9F is an illustration of a portion of the delivery device modifiedto have circumferentially displaced apertures to enable the proximal anddistal portions of the fixation members to be released in sequence;

FIG. 10 is a diagrammatic side elevation of the sensor assembly of FIG.2 deployed in a pulmonary artery of a patient;

FIG. 11 is a diagrammatic illustration, in plan, of the deployed sensoras seen along the line 11-11 of FIG. 10;

FIG. 12 is an isometric illustration of another embodiment of a sensorassembly having a modified fixation member;

FIG. 13 is a diagrammatic plan view of the embodiment of the sensorassembly of FIG. 12 as viewed from its underside, showing the connectionbetween the fixation member and sensor housing;

FIG. 14 is a an enlarged illustration of the underside of the sensorassembly of FIG. 13 viewed from a different angle;

FIG. 15 is a diagrammatic end view of the sensor assembly of FIG. 12 asseen along the line 15-15 of FIG. 13;

FIG. 16 is a diagrammatic end view of the device of FIG. 12 illustratingthe position of the device when deployed in a vessel;

FIG. 17 is an isometric illustration of another embodiment incorporatingaspects of the invention;

FIG. 18 is an end view of the embodiment of FIG. 17;

FIG. 19 is a side view of the embodiment of FIG. 17

FIG. 20 is a diagrammatic illustration of the embodiment of FIG. 17deployed in a blood vessel;

FIG. 21 is an illustration of a sensor assembly having a tubularfixation member with the sensor housing attached in accordance with oneaspect of the invention; and

FIG. 22 is an enlarged illustration of a portion of the sensor assemblyof FIG. 21 showing the connection between the sensor housing and thefixation member.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The terms “distal” and “proximal” are used in the following descriptionwith respect to a position or direction relative to the treatingclinician. “Distal” or “distally” are a position distant from or in adirection away from the clinician. “Proximal” and “proximally” are aposition near or in a direction toward the clinician.

FIG. 1 illustrates, diagrammatically, a patient with implanted medicaldevices including a sensor assembly 10 implanted, for example, in thepatient's pulmonary artery 12 through which blood flows from the heart14 to the lungs, and another device, such as a pacemaker, defibrillatoror the like, indicated generally at 16. For purposes of thisdescription, knowledge of cardiovascular anatomy is presumed and detailsare omitted except to the extent necessary or desirable to explain thecontext of the invention. The device 16 may have a number of leads 18,20, 22 that are placed in electrical contact with selected portions ofthe cardiac anatomy in order to perform the functions of the device 16as is well known to those skilled in the art. The device 16 also mayhave wireless capability to receive and transmit, by telemetry, signalsrelating to operation of the device. The device 16 may link wirelesslyto an external device such as a programmer 24 or to another implanteddevice such as a sensor 26 of the sensor assembly 10. For sake ofclarity, sensor assembly 10 is shown without a fixation member inFIG. 1. See FIGS. 10 and 20. The sensor 26 also may communicatewirelessly with an external receiver 28 to provide in vivo data forselected physiological parameters to an external site to informclinicians of the patient's status.

FIGS. 2-6 illustrate one embodiment of a sensor assembly 10 adapted forminimally invasive placement in a patient's blood vessel, the assemblybeing shown in its expanded configuration. The sensor assembly 10includes a sensor 26 and a fixation member 30 to which the sensor isattached. The fixation member 30 and sensor 26 are arranged to enablethe assembly to be collapsed to a low profile to enable it to be carriedby a delivery catheter and navigated to a deployment site where it canbe released. Upon release, the fixation member expands into engagementwith the wall of the blood vessel to secure it in place. The sensor 26is attached in a manner that when the fixation member 30 is placed thesensor 26, and particularly the sensing element 32 of the sensor 26, arespaced from the wall of the vascular lumen to minimize adverseobstruction to blood flow through the lumen and to position the sensingelement 32 of the sensor 26 to be fully exposed to the blood in thevessel, without obstruction from the body of the sensor or the vesselwall.

The sensor 26 includes a housing 34 preferably formed in two sections36, 38, one of which (36) may contain a battery for powering theelectronics and sensor components contained in the other section 38. Thehousing 34 preferably is of elongate, cylindrical shape with roundedends and a cylindrical sidewall extending between the ends. This shapeis considered to present low resistance to blood flow. Other housingconfigurations may be employed, however. The sections are formed from abiocompatible material that can be hermetically sealed when the sections36, 38 are joined. A number of such biocompatible materials may beemployed, as will be understood by those familiar with the art,including metals and biocompatible plastics. For example, the sections36, 38 may be formed from unalloyed titanium with an American Societyfor Testing and Materials (ASTM) grade 1 to grade 4 or an alloyedtitanium (grade 5) that includes aluminum and vanadium. For embodimentsin which the sections are metal, the metal should have sufficientmalleability to facilitate secure attachment of the housing 34 to thefixation member 30 by crimping, as described in more detail below. Thehousing as well as some portions of the fixation member may beencapsulated in a biologically inert dielectric barrier material such asa film of silicone or poly(p-xylylene) polymer sold under the trademarkPARYLENE. Those portions of the housing or fixation member that areintended to serve as poles for intra-body wireless communication (e.g.,to transmit or receive RF signals) may remain uncovered.

The fixation member 30 is wire-like and, in this aspect of theinvention, is configured to lie substantially in a single plane,indicated generally at 40 in FIG. 3. In the embodiment of sensor 10illustrated in FIGS. 2 and 3, sensor 26 is mounted to fixation member 30such that sensor housing 34 and sensing element 32 are intersected byplane 40. In an exemplary embodiment the fixation member 30 may beformed from a highly elastic, biocompatible alloy capable of formingstress induced martensite (SIM). Nitinol (TiNi) is an example of suchmaterials that are also referred to as being “pseudoelastic” or“superelastic.” The fixation member 30 shown includes a pair oflongitudinally spaced loops 42 connected by an elongate linearattachment strut 44. The loops 42 are spaced apart sufficiently toreceive and embrace the sensor 26 with the sensor extending lengthwisealong the attachment strut 44. The sensor 26 is attached to theattachment strut 44 to symmetrically lie along the single common plane40. The fixation member 30, including the attachment strut 44 may beformed from a sheet of material by laser cutting or electrochemicaletching or other fabricating techniques known in the art. The resultingfixation member 30 has a substantially uniform thickness and is formedas a single, integral piece. The wire-like elements that make up theloops 42 and the attachment strut 44 may have a non-circular crosssection that may be square or rectangular.

As illustrated in FIGS. 5-7, the arrangement for attaching the sensor 26to the strut 44 includes several pairs of tabs 46, 48 formed integrallywith the housing 34 preferably on the housing section 36 that containsthe battery and at a location that is diametrically opposite the sensingelement 32. The pairs of tabs 46, 48 are aligned longitudinally andcooperate to define a longitudinally extending channel 50 that isnon-circular in cross-sectional shape (preferably rectangular) and isreceptive to attachment strut 44 of the fixation member 30. The width ofchannel 50 is selected to receive the rectangular cross section ofattachment strut 44 in a snug fit to prevent housing 34 from rotatingabout the axis of attachment strut 44. The tab pairs 46, 48 may bearranged as shown, with two inner pairs 46 disposed between the twoendmost pairs 48. The inner tabs 46 extend outwardly from the body ofthe sensor a distance greater than the outward extension of the endmosttabs 48. The inner tabs 46 will extend beyond the height 52 of theattachment strut 44 so that when strut 44 is seated in channel 50 theends of the inner tabs 46 may be crimped over strut 44 as shown in FIG.5. The endmost tab pairs 48 may extend less than the crimping tabs 46and serve only to define the channel 50 and guide attachment strut 44into the channel 50. As shown in FIG. 6, endmost tab pairs 48 need notextend beyond the exposed surface 54 of strut 44. The tabs may beprovided with detent elements 55 (FIG. 6) to facilitate alignment withthe tabs of a crimping tool having complementary elements (not shown).

By way of dimensional example the thickness of fixation member 30 may beof the order of about 0.012 inch, including attachment strut 44. Thewidth of channel 50 may be of the order of about 0.0125 inch. The heightof strut 44 may be of the order of about 0.015 inch. The crimp tabs 46may be slightly higher than the height of the attachment strut by anamount to enable their ends to be crimped over the attachment strut. Thetabs may be about 0.008 inch thick and 0.040 inch long. The guide tabs48 may be somewhat longer than the crimp tabs 46, for example, about0.055 inch, and their height may be about the same as that of the strut44. As illustrated in FIG. 7, the battery segment 36 of the housing, onwhich the tabs are mounted, may include a slightly raised pad 47 thatdefines the bottom of channel 50 and from which the tabs extend. Sensorhousing 34 may be spaced from strut 44 by the thickness of raised pad47. It should be understood that the arrangement of tabs, includingtheir number and dimensions may be varied as desired to provide anarrangement by which a linear attachment strut, non-circular in crosssection, may be inserted transversely into the channel 50 and crimped inplace by tabs. It should be understood, however, that although theforegoing embodiment is presently preferred, other connectorarrangements might be employed while still retaining other aspects ofthe invention described herein. For example, such connector arrangementsmay include tubular connectors as described in provisional applicationSer. No. 61/408,073, filed Oct. 29, 2010 and entitled Medical DeviceFixation Attachment Mechanism, the disclosure of which is incorporatedherein by reference.

FIG. 8 illustrates the low profile configuration of the sensor assemblyin which fixation member 30 with sensor 26 attached is compressed to anarrower shape having a smaller effective cross section in which it maybe mounted to and delivered by a delivery device 66 to an intendeddeployment site such as the pulmonary artery. In this configuration eachof the loops 42 is compressed from its relaxed, expanded shape to a moreelongated shape defined by loop segments 56 that are drawn more closelyparallel to each other. Although fixation member 30 is formed from asuperelastic material such as superelastic nitinol having the ability toundergo extreme strain without permanent deformation, it is desirable toshape the loops 42 to reduce the risk of plastic deformation whencompressing the loops to the low profile delivery configuration. To thatend we have found that forming the loops 42 in a teardrop shape resultsin a reduced strain in the region of the juncture of the loop 42 withthe attachment strut 44. Thus, as shown in FIG. 2 each loop 42 may beconsidered as having loop segments 56A and 56B connected to each otherby a bight segment 58 that circumscribes an arc of approximately 180degrees. Loop segment 56B may be considered as a linear extension ofattachment strut 44. Loop segment 56A is generally linear and isattached to the inner region of the loop segment 56BA at a junction 60.The angle 62 made by the loop segments 56A and 56B at the junction 60 issubstantially smaller than the angle subtended by the arc of the bight58 resulting in teardrop shaped loop 42. In the embodiment illustratedin FIG. 2, junction angle 62 may be of the order of about 90 degrees,less than the arc of about 180 degrees defined by the bight.Alternatively, in the embodiment illustrated in FIG. 10, loop segment56A may include a portion having a curve that is reversed from that ofbight 58 such that junction angle 62 may be of the order of about 45degrees. In the embodiment of FIG. 10, bight segment 58 may circumscribean arc greater than 180 degrees and loop segment 56A may have nostraight portion. When fixation member 30 is compressed to bring theloop segments 56A, 56B into a more parallel low profile configurationthe loop segment 56A will be bent through a relatively smaller arc ofabout 90 degrees or less although at a relatively small radius. Bylimiting the arc through which the loop segment 56A must bend the riskof plastic deformation in the region of the junction is reduced. Whilethe extremity 64 of the bight segment 58 of the loop 42 will bendthrough a greater arc, the radius of the compressed bend at theextremity 64 is greater than that at the junction 60 as shown in FIG. 8.

FIGS. 9A-9E depict, diagrammatically and in fragmented illustration, anexample of a delivery device shown as in FIG. 8 that may be used todeliver and deploy the sensor assembly 10 in a selected vessel. Deliverydevice 66 may also be used to deliver sensor assembly 87, describedfurther below. The delivery device 66 is in the form of an elongateouter tubular catheter shaft 68 having proximal and distal ends 70, 72with the sensor assembly 10 mounted on the outer surface of the distalregion of the catheter 68 in its compressed, low profile configuration(FIG. 8). The shaft 68 should be formed from a material and dimensionedto have sufficient flexibility to be navigated through the patient'svasculature to the intended deployment site. The device may be guidedthrough a guide catheter or in association with a guide wire, as is wellknown to those skilled in the art. The sensor assembly may be releasablyretained on the catheter shaft by an arrangement of rotatable helicalretention elements 74 attached to a rotatable inner shaft 76 (FIGS.9A-9F) housed within the catheter 68. Each of the helical retentionelements is disposed about the shaft 76 and has one end 77 secured tothe shaft 76, as by adhesive 79 or other suitable means, and another,free end 78. The catheter shaft 66 has at least one exit aperture 80 andpreferably two apertures 80, also a reentry aperture 81 through whichthe free end 78 of each retention element 74 can pass. As shown in FIGS.9C and 9D the shaft 76 can be rotated to enable a retainer portion,including the free end 78 of the retention coil 74 to protrude out ofthe exit aperture 80 and pass over a compressed loop 42 of the fixationmember to retain the loop on the catheter until deployment. The helicalcoil is sufficiently flexible and resilient so that the turn that passesover the fixation member can resiliently increase in diameter toaccommodate the fixation member. Providing the second aperture 81through which the free end 78 of the retention member may reenter thecatheter shaft may add to the security by which the sensor assembly isretained on the catheter shaft. A helical retention element 74 isprovided for each of the proximal and distal loops 42.

The delivery device may be advanced through a guide catheter or sheath67 (FIG. 9E) that may be retracted to expose the sensor assembly. Whenthe delivery catheter has been positioned as desired and the sensorassembly is to be deployed, the shaft 76 is rotated, as by a controlknob 75 attached to the proximal end of the rotatable shaft (FIG. 8), torotate the helical retention elements 74 and retract the retainerportions and free ends 78 into the catheter shaft, thus releasing thesensor assembly. As the sensor assembly is released it self-expands toits expanded configuration within the vessel. The retention elements 74may be arranged to release both the proximal and distal loopssimultaneously or may be arranged to release one before the other, forexample, to release the distal loop 42 first, to observe its positionand confirm it is in the intended location in the vessel. That may allowfor repositioning the distal loop 42, after which the proximal loop 42may be deployed. Repositioning may be accomplished by advancing theguide catheter or sheath distally to recapture the distal loop 42 whilethe unreleased retention member retains the position of the sensorassembly. The recaptured sensor assembly then may be repositioned andredeployed. It may be noted that although the loop segments 56A and 56Bare approximately parallel when in their low profile configuration theydo diverge slightly towards their respective bights 91. The divergencemay cause a wedging effect to secure the proximal retention member andthe proximal loop as the guide catheter or sheath is advanced distallyto recapture the sensor assembly.

The sequence of release of the loops 42 may be determined by therelative circumferential location of the retention elements 74 and theirspiral ends 78 and the location of the openings 80, 81 or by arrangingthe helical retention coils 74 on shaft 76. Alternately the releasesequence can be varied by providing one of coils with more turns betweenits free end and the turn that engages its associated fixation memberloop than the other of the coils. The retention member with the fewernumber of free turns will be first to release its associate loop. Forexample, by arranging the distal coil to have fewer free turns than theproximal coil, the distal loop 42 will be released first enabling theclinician to determine if the sensor is properly positioned.

The delivery device may be advanced to the intended deployment site byadvancing it through a guide catheter or may be guided over a guide wirein an over-the-wire system, both of which are familiar to those skilledin the art. It should be understood that delivery device 66 is only oneexample of a delivery system for sensor assembly 10. Other types ofdelivery systems can be utilized, such as an outer sheath (not shown)slidably disposed around the sensor assembly to constrain the sensorassembly in its low profile configuration until a pusher mechanismejects the sensor assembly from the distal end of the sheath. It shouldbe noted that the superelastic construction of the fixation memberenables it to be elastically distorted from its planar expanded shape toa shape adapted to fit onto or within a delivery catheter.

FIGS. 10 and 11 illustrate, diagrammatically, the positioning of thesensor assembly 10 in a pulmonary artery 12. The human main pulmonaryartery 12 is relatively short and often has a lumen 82 that tapers inthe direction of blood flow. The degree of taper may vary from patientto patient, with patients suffering from chronic heart failure tendingto have more severe taper with higher pulmonary blood pressures. Themain pulmonary artery branches into left and right pulmonary arteriesand whether the clinician will elect to place a device in the mainartery or one of the branches of the pulmonary artery tree will dependon the anatomy and condition of the particular patient among otherfactors. When deploying the sensor assembly 10 the delivery catheter ispositioned so that the more distal of the loops 42 will be located inthe selected portion of the selected artery in which the distal loop 42will expand to engage the luminal surface 84, 86 of the vessel wall withonly sufficient force to hold the sensor assembly in place. Loop 42 isexpected to apply little more than the force that is appropriate to holdthe device in place without applying excessive force to that surface.The fixation member 30 should be constructed to apply light butsufficient force to the vessel. The forces to be applied aresubstantially less than those associated with the placement of vascularstents in which the objective is to press against the vascular wall withsufficient force to provide scaffolding support for the vessel wall. Bycontrast, the present invention is intended merely to maintain thesensor assembly 10 in the vessel without migrating upstream ordownstream while supporting the sensor 26 in its intended position andorientation. When the sensor assembly 10 is deployed the fixation member30 expands to its single plane with the most distal loop expanding to adimension to engage the luminal wall of the vessel. Regardless of theorientation of the assembly during deployment the loop 42 will seatitself to engage substantially diametrically opposite surfaces 84, 86 ofthe vessel wall with the attachment strut 44 extending along one ofthose surfaces. In that deployed position, the sensing element 32 of thesensor 26 will be oriented to face the center of the lumen to be exposedfully and without obstruction to blood flow in the lumen. It may benoted that in tapered vessels such as in a sharply tapered pulmonaryartery the proximal of the loops 42 may not engage diametricallyopposite surfaces of the vessel wall. In such circumstances the bloodflow in the artery may cause the sensor assembly to pivot about itspoints of contact of the distal loop 42 with the vessel wall assuggested in phantom in FIG. 11. While that may cause the proximal loop42 to swing sideways the proximal loop 42 will engage the vessel wall tomaintain the sensor 26 spaced from the wall.

The sensor assembly is placed by advancing the delivery catheter to aselected location within the vessel and deploying the sensor assembly sothat it remains secure in the vessel at that location. To that end, theexpanded dimensions of the fixation member are selected to be slightlygreater than a predetermined transverse dimension (“effective diameter”)of the vessel into which it is to be placed. By way of example, for adevice formed from a superelastic material intended for placement in apulmonary artery, the fixation member preferably may have an expandeddimension about twenty percent greater than the effective diameter atthe selected location of the artery. Where the device is to be placed.While the dimensions and anatomies of the pulmonary arterial treenecessarily vary among patients, particularly heart failure patients, weconsider that many, perhaps most candidates for such devices are likelyto have a region of the arterial tree with an effective diameter ofabout ten millimeters. Therefore, and by way of example, the fixationmember preferably has a distal loop with a relaxed height 57 (FIG. 2) ofabout twelve millimeters. When placing the device, the cliniciandetermines and selects a suitable location in the patient's arterialtree with an effective diameter of about ten millimeters, navigates thedelivery catheter to locate the sensor assembly at that location andthen releases the assembly so that it expands into sufficient engagementwith the vessel to prevent migration. The characteristics of thesuperelastic material and dimensions of the fixation member should beselected so that the force applied by the deployed fixation member tothe artery wall causes no adverse trauma while fixing the device inplace.

FIGS. 12-16 illustrate another example of a sensor assembly 87 embodyingthe invention in which the fixation element 88 and sensor 26 arearranged to support the sensor more centrally in the vascular lumen. Inthis embodiment the attachment strut 90 (FIG. 14) may be shorter thanthat of the embodiment of FIG. 2 and the sensor 26 is supported to oneside of the plane 40 (FIG. 15). Additionally, the teardrop-shaped loops92 may be configured to have a smaller junction angle 62 because thejunctions 60 of the loops 92 can be located closer to each other. Asdescribed above, the reduced junction angle 62 reduces the amount ofstrain on the junction 60 when compressing the fixation member to a lowprofile configuration. FIG. 16 illustrates, diagrammatically, the mannerin which the fixation member 88 supports sensor 26 when deployed in avessel, for example, a pulmonary artery 12. As with the example of FIG.2, upon release in the artery, fixation member 88 expands to engage thevessel wall at approximately diametrically opposed locations. In thisembodiment sensor 26 is oriented to the side of the single common plane40 of the fixation member 88 (FIG. 15) and will be supported to extendlongitudinally of the vessel and approximately in the middle of thevessel lumen. The outermost extremities 91 of the loops 92 preferablyextend sufficiently beyond the ends of sensor 26 so that even if theproximal end of the assembly shifts laterally toward the wall of thevessel, the proximal one of the loops 92 will contact the wall beforethe sensor to maintain the sensor spaced apart from the wall. The mannerof attachment of sensor 26 to fixation member 88 may be substantiallythe same as described above. Attachment strut 90 in this embodiment needonly be long enough to be received in channel 50 of the sensor. Thesensor assembly may be mounted to a delivery device as described above.

FIGS. 17-20 illustrate a further example of a sensor assembly 94embodying principles of the invention. In this example the fixationmember 96 includes a pair of oval loops 98 disposed in side-by-siderelation joined by a common attachment strut 100. Each loop 96 includesan arcuate bight 102 at each of its ends and a longitudinally extendingside strut 104. In their expanded configuration the loops 96 liesubstantially in a single plane 106. When deployed in a vessel, assuggested diagrammatically in FIG. 20 the side struts 104 of the ovalloops 98 may engage diametrically opposed portions of the vessel walland support the sensor capsule 26 substantially centrally in the lumenof the vessel. In this embodiment sensor 26 is oriented to the side ofthe single common plane 106 of the fixation member 88 (FIG. 18) and willbe supported to extend longitudinally of the vessel and approximately inthe middle of the vessel lumen. A diagrammatic end view of the device ofFIG. 20 illustrating the position of the device when deployed in thevessel would appear similar to the illustration shown in FIG. 16.Because struts 104 of fixation member 96 engage opposed elongate,longitudinally extending portions of the luminal surface of the vesselwall, the position and orientation of the fixation member 96 and sensorhousing 26 provide additional resistance to reorientation of sensorassembly 94 to a canted position. Thus, the length of fixation member 96may be reduced in this example to be only slightly longer than thesensor capsule 26 FIG. 19. Here, too, the sensor assembly may be mountedto a delivery device configured to accommodate the fixation member 96 ofthe sensor assembly

Certain aspects of the invention also may be incorporated into othersensor assemblies in which the fixation member may take forms other thanthe single plane examples described above. For example, FIGS. 21 and 22illustrate a sensor assembly 108 with a fixation member 110 having agenerally tubular expanded shape that is compressible to a low profiletubular shape to facilitate catheter delivery. The fixation member isdefined by a plurality of links 112 arranged to define expandable cells114. Such tubular fixation elements may be fabricated using any of avariety of well known techniques commonly employed for fabricatingtubular stents, for example, by laser cutting the pattern of thefixation member from a tube of the selected material. In accordance withone aspect of the present invention such tubular fixation members 110may incorporate at least one longitudinally extending, linear strut 116having a non-circular cross section and integrally joined at its ends toother elements of the fixation member 110 so that it may serve as anattachment strut to which a sensor capsule 26 may be secured. As shownin the drawings the sensor capsule 26 may be the same as that describedabove in connection with the example of FIG. 2 with tabs arranged todefine a channel 50 receptive to the linear attachment strut 116.Assembly of the fixation member 110 and sensor capsule 26 is the same,namely, inserting the strut 116 transversely into the channel 50 andcrimping the crimp tabs to secure the parts together. The tubularfixation member may be delivered to the deployment site by a catheterhaving a tubular chamber at its distal end adapted to receive the sensorassembly in a collapsed, low profile configuration similar to suchdelivery systems used with vascular stents. The fixation member 108 mayinclude a tether 118 by which the device may be drawn into an opening atthe distal end of the delivery catheter while progressively collapsingthe fixation member to a low profile delivery configuration.

It should be understood that the foregoing examples of embodiments ofthe invention are illustrative only and that other embodiments,modifications and equivalents may be apparent to those skilled in theart that nevertheless are within and embody the principles of theinvention.

We claim:
 1. A delivery device for implanting a medical device thatincludes at least one fixation member adapted to fix the position of themedical device within a lumen of a human body, the fixation member beingexpandable from a low profile configuration in which it can be passedinto the lumen to an expanded configuration in which it engages a wallof the lumen to maintain the medical device in place, the deliverydevice comprising: a tubular outer shaft; an inner shaft rotatablydisposed in the outer shaft; and at least one retention member having afree end, the retention member being secured to and rotatable with theinner shaft, each retention member being associated with an exitaperture formed in the outer shaft to enable a retainer portion of theretention member, that includes its free end, to protrude outwardlythrough its associated exit aperture, the retainer portion of theretention member being adapted to extend circumferentially about atleast part of the exterior of the outer shaft, the retainer portion ofthe retention member being retractable into the outer shaft in responseto rotation of the inner shaft; whereby the fixation member of themedical device may be retained on the tubular shaft in its low profileconfiguration by the outwardly protruding retainer portion and may bereleased to expand to its expanded configuration upon retraction of theretainer portion in response to rotation of the inner shaft.
 2. Thedelivery device of claim 1 wherein the retention member comprises ahelical coil disposed about the inner shaft and secured to the innershaft at a location longitudinally spaced from the free end of the coil.3. The delivery device of claim 2 wherein the retention member is bondedto the inner shaft.
 4. The deliver device of claim 1 further comprising:the outer shaft having a reentry aperture associated with each exitaperture, the reentry aperture being circumferentially spaced from itsassociated exit aperture and being positioned to receive the free end ofits associated retention member to retain the free end of the retentionmember upon reentry into the outer shaft.
 5. The delivery device ofclaim 1 wherein the delivery device has at least two retention membersat spaced locations along its length and an exit aperture associatedwith each of the retention members whereby the retention members mayengage different portions of the fixation member.
 6. The delivery deviceof claim 5 wherein the outer shaft has a reentry aperture associatedwith each of the exit apertures, each reentry aperture beingcircumferentially spaced from its associated exit aperture and beingpositioned to receive the free end of its associated retention member toreenter the outer shaft and retain the free end of the retention member.7. The delivery device of claim 5 wherein the exit apertures arearranged in sufficiently circumferentially spaced positions on the outershaft to cause the different portions of the fixation member to bereleased in a predetermined sequence.
 8. The delivery device of claim 5wherein the retention members are constructed and arranged to releasedifferent portions of the fixation member in a predetermined sequence.9. The delivery device of claim 5 wherein each retention membercomprises a helical coil disposed about the inner shaft and secured tothe inner shaft at a location longitudinally spaced from the free end ofthe coil and further comprising: one of the coils having a greaternumber of turns between its free end and the location where the retainerportion engages its associated portion of the fixation member thananother of the coils, whereby the retention member with the fewestnumber of such turns will release its associated portion of the fixationmember first.
 10. The delivery device of claim 8 wherein the fixationmember has proximal and distal portions and wherein the retentionmembers are arranged to release the distal portion of the fixationmember before releasing the proximal portion.
 11. The delivery device ofclaim 1 wherein the medical device has a module adapted to perform adiagnostic or therapeutic function, the fixation member having a portionprotruding beyond an envelope of the module, the protruding portionbeing mounted to the outer shaft and being releasably retained thereonby the retainer portion of the retention member.
 12. The delivery deviceof claim 10 further comprising: an outer sheath disposed about the outershaft, the outer sheath being proximally retractable to expose themedical device, the outer sheath being advanceable distally to recapturea released distal portion of the fixation member to enable the medicaldevice to be repositioned and redeployed.
 13. The delivery device ofclaim 12 wherein the outer sheath comprises a guide catheter.
 14. Thedelivery device of claim 1 wherein the retention member is formed from asuperelastic material.
 15. A method of delivering and deploying amedical device that includes at least one fixation member adapted to fixthe position of the medical device within a lumen of a human body, thefixation member being expandable from a low profile configuration inwhich it can be passed into the lumen and an expanded configuration inwhich it engages a wall of the lumen to maintain the medical device inplace, the method comprising: mounting the medical device on a tubularcatheter shaft with the fixation member in its low profileconfiguration; extending a retention member from within the cathetershaft through an aperture in the catheter shaft outwardly andcircumferentially about the shaft and a portion of the low profilefixation member to retain the medical device on the shaft; navigatingthe catheter shaft and mounted medical device to an intended deploymentsite in a body lumen of a patient; and retracting the retention memberinto the catheter shaft to release the fixation member from the shaftand enabling the fixation member to expand to its expanded configurationin the lumen.